Distribution of Pt single atom coordination environments on anatase TiO2 supports controls reactivity

Single-atom catalysts (SACs) offer efficient metal utilization and distinct reactivity compared to supported metal nanoparticles. Structure-function relationships for SACs often assume that active sites have uniform coordination environments at particular binding sites on support surfaces. Here, we investigate the distribution of coordination environments of Pt SAs dispersed on shape-controlled anatase TiO2 supports specifically exposing (001) and (101) surfaces. Pt SAs on (101) are found on the surface, consistent with existing structural models, whereas those on (001) are beneath the surface after calcination. Pt SAs under (001) surfaces exhibit lower reactivity for CO oxidation than those on (101) surfaces due to their limited accessibility to gas phase species. Pt SAs deposited on commercial-TiO2 are found both at the surface and in the bulk, posing challenges to structure-function relationship development. This study highlights heterogeneity in SA coordination environments on oxide supports, emphasizing a previously overlooked consideration in the design of SACs.


Supplementary Discussion Ⅰ, Summary of previous literature on the validation of near 100% Pt dispersion on Pt(0.025)/TiO2-commercial sample.
It was demonstrated in our prior studies that Pt can be exclusively atomically dispersed on TiO2 supports by using small oxide nanoparticles as supports and depositing less than one Pt atom per particle. 1,2In this study, TiO2-commercial represents anatase TiO2 particles with a diameter of ~5 nm (surface area, 290 m 2 /g, US Research Nanomaterials), and 0.025 wt.% of Pt corresponds to ~0.2 Pt atom per TiO2 particle.
Supplementary Figure 8a shows CO-IR spectra of Pt/TiO2-commercial with different Pt loadings (0.025, 0.05 and 1 wt.%) collected after an in-situ reduction with 10% H2 at 250 o C for 1 hr.At a Pt loading of 0.025 wt.%, the predominant CO stretching band is observed at ~ 2112 cm -1 which was attributed to CO bound to Pt SA. 1,2 In addition, another broader CO stretching band is observed at the lower wavenumbers, in the range of 1900-2100 cm -1 , at the higher Pt loading, which was attributed to CO bound to Pt NP. 2 When purged with Ar at 25 o C, the CO-IR band intensity at ~ 2112 cm -1 decreased rapidly (Supplementary Figure 8b-c), whereas the intensity of the CO-IR band in the range of 1900-2100 cm -1 did not (Supplementary Figure 8c).This agrees with previous reports that CO binds weakly to Pt SA but strongly to Pt NP.Supplementary Figure 8 shows that Pt is mostly atomically dispersed on TiO2-commercial at a Pt loading of 0.025 wt.%.Consistent with our conclusions from CO-IR spectra, only Pt SAs could be found in the STEM images of Pt(0.025)/TiO2-commercial (Supplementary Figure 7).Supplementary Figure 9. XPS spectra of TiO2-trucated bipyramid with different Pt loading, 0 wt.%, 0.05 wt.%, and 0.25 wt.%.The broad Ti 3s energy loss peak from pure TiO2 supports and the shorter mean free path of Pt than Ti hinder the observation of Pt 4f XPS signal at a low Pt content of 0.05 wt.%.This signal was enhanced after increasing the Pt content to 0.25 wt.%.The morphology of Pt(0.05)/TiO2-truncated bipyramid and Pt(0.25)/TiO2-truncated bipyramid are shown in Supplementary Figure 6 and Supplementary Figure 14 It should be noted that PtO2 is sensitive to low energy X-rays (30 watts), leading to a shift in XPS peak towards the higher energy, as shown by the fitted orange peaks in PtO2.However, despite this sensitivity, a large amount of PtO2 still remains after X-ray exposure, as evidenced by the fitted red peaks.

Supplementary Discussion Ⅱ, Estimation of the Pt oxidation state using XPS spectra.
Based on the experimental XPS spectra of Pt metal and PtO2 (IV) shown in Supplementary Figure 11, along with the reference Pt 4f binding energy in Pt metal foil, PtO (II), and PtO2 (IV) as mentioned in the literature, 3-5 the Pt 4f7/2 binding energies for Pt 0 and Pt 2+ and Pt 4+ are determined to be 71.0,72.4,74.3 eV, respectively.These values have been marked as three blue dots in the figure displayed below.It is observed that these three dots show a linear relationship between Pt oxidation state and Pt 4f7/2 binding energy, represented in a blue dot line, indicating that Pt 4f7/2 binding energy increases linearly with the rise of Pt valence state.
In Figure 2, the Pt 4f7/2 binding energies of Pt(0.25)/TiO2-truncated bipyramid and Pt(0.25)/TiO2-nanosheet are reported as 72.5 eV and 73.2 eV.By applying these binding energies to the previously established linear fitted curve, it is determined that the corresponding Pt oxidation states for Pt(0.25)/TiO2-truncatedbipyramid and Pt(0.25)/TiO2-nanosheet are approximately +1.9 and +2.8, respectively.The higher Pt valence state of Pt SAs in Pt(0.25)/TiO2-nanosheet compared to Pt(0.25)/TiO2-truncated bipyramid suggests that Pt SAs located at the subsurface of the TiO2(001) in the nanosheet are more cationic and more coordinated with the oxygen atoms within the TiO2 lattice, as compared to those on the TiO2(101) surface.13.HAADF STEM images of Pt(0.25)/TiO2-nanosheet.Abundant Pt SAs are found (marked by yellow dotted circles) on the specifically oriented (001) surface.The observed number density of Pt SA is estimated to be ~0.03atom/nm 2 .This value is lower than the nominal surface coverage of 0.14 atom/nm 2 , as one HAADF STEM image can only detect Pt SAs that exist in TiO2 within the depth of field, and may not capture all Pt SAs throughout the entire thickness of TiO2. 1. Atomic ratios of Ti, F and O atoms.Atomic ratios of Ti, F, and O in (1) TiO2-nanosheet, (2) Pt(0.25)/TiO2-nanosheet, (3) Pt(0.25)/TiO2-nanosheet after reduction with H2, and (4) Pt(0.25)/TiO2-truncated bipyramid are estimated from XPS spectra.( 1), ( 2) and (4) samples were oxidized with air at 300 o C before collecting the spectra.

Supplementary Figure 10 .
, respectively.Pt 4d XPS spectra of Pt(0.25)/TiO2-nanosheet before and after Ar sputtering.(a) Spectra before and after sputtering with Ar1000 clusters at 10 keV for 10s.(b) Magnified view of Pt 4d spectrum after sputtering and corresponding fitted spectrum.The increase in the Pt signal after Ar sputtering compared to its initial level indicates that a large fraction of Pt is located in the bulk of TiO2-nanosheet.Supplementary Figure 11.Pt 4f XPS spectra of metallic Pt and PtO2 with Pt valence states of 0 and +4.

Supplementary Figure 17 .
F 1s XPS spectra of TiO2-nanosheet with and without Pt.The peak located at 684.0-684.3eV corresponds to the presence of F on the TiO2 surface with TiOF2 or Ti-F bonding.Another peak located at 682.0 eV represents the presence of surface F bonded on TiO2 surface as TiOFx (x<2).Supplementary Figure 20.CO-IR spectra of (a) Pt/TiO2-nanosheet and (b) Pt/TiO2-truncated bipyramid with different Pt loading (0.05, 0.25 and 1 wt.%).Spectra were collected after flowing 10% CO at 35 o C for 10 min, and the gas-phase CO signal was manually removed from the spectra.Before flowing CO, samples were reduced with 10% H2 at 250 o C for 1 hr, followed by purging with Ar at 250 o C for 30 min.Supplementary Table 2. Summary of the FWHM and the centroid of the IR band from CO bound to Pt SA dispersed on anatase TiO2 support from our work and those from literature.